When an aircraft wing, for example, is constructed as a so-called monocoque structure, the wing box comprises an upper shell or skin, a lower shell or skin, a leading edge spar, a trailing edge spar, and ribs. The shells or skins comprise wing skin sections extending in the direction of the wing span. These shells are stiffened by longitudinal stringers or spars extending in the wing span direction and by ribs extending substantially perpendicularly to the wing span direction. The spars are in principle flat structural components while the shells or skins have a three-dimensionally curved configuration in accordance with the aerodynamic profile of an aircraft wing. The ribs arranged inside the wing box extend from spar to spar and are connected along their long edges with the skin or shells. The connection between the spars and the ribs or between the shells and the spars and ribs are conventionally formed by all purpose rivets or by threaded rivets having a set head and a closure head, whereby the closure head is formed on the inside of the structure by a swage tool or by screwing the rivets into the structure. These rivets permit the production of cost efficient connections having consistently reproducible static characteristics.
In order to achieve a structure surface that is as smooth as possible, it is customary to primarily use countersunk rivets. If the structure has a sufficient profile thickness and a relatively small profile depth of the wing or tail unit, then there are no problems in the assembly of the shells with the ribs and spars, because the accessibility required for the rivet setting swage tool to the rivet location from the inside is provided by respective handholes or manholes. In other words, proper accessibility is provided as long as the wing thickness measured vertically and perpendicularly to the wing span, is sufficient for the tool insertion even if the wing depth or profile depth measured horizontally is relatively small. These considerations apply equally to wings and components of the tail assembly. However, problems occur if a wing box or tail unit box has only a small profile thickness while simultaneously having a relatively large profile depth, as is for example the case for aircraft capable of supersonic speeds. In such instances the accessibility for the riveting operation is very limited so that the attachment of the skin sections to the spars and ribs becomes very difficult. The substantial use of blind rivets does not provide a solution to the problem, because the type of blind rivets that may be used for the intended purpose have two critical disadvantages. First, if special rivets are used, they are too expensive. Second, general purpose rivets, though less expensive, are limited in their use by government regulation.
Another method for producing an aerodynamic structure having a very small profile thickness, for example for an elevator assembly, is described in a book by Michael C. Y. Niu entitled "Air Frame Structural Design", published by Technical Book Company, 1991, Los Angeles, page 368. FIGS. 10.2.8 or 10.2.10(a) in this publication show a tail assembly in the form of a so-called flying tail. In principle, a flying tail comprises two shells sandwiching a honeycomb core therebetween having the profile or configuration of a fin or flap. The shells forming the outer skin are secured to the honeycomb core by adhesive bonding. This type of construction of a very thin aerodynamic surface or rather body in principle does not require any riveting work so that the inner space of the fin or flap does not need to be accessible during production. This fact is a manufacturing advantage which, however, must be compared to the disadvantage that in operation the structure such as a fin is exposed to the normal loads and to additional loads caused by the closed construction. Such additional loads occur due to differences between the pressure inside the structural component and the pressure outside the structural component. This differential pressure can cause leaks so that condensation water may form inside of the honeycomb core, whereby in turn the adhesive bonding connections may be damaged.
Since the present structure shall be successfully usable at supersonic speeds, the structure must have a sufficient fatigue strength due to the expected dynamic loads. In connection with aerodynamic surfaces intended for use at supersonic speeds, it is further very important that the aerodynamically ascertained profile thicknesses must be met with a very high accuracy during manufacture.